sx: step 8 — non-exhaustive match warnings

Emit a warning when a `match` expression on an ADT value misses one
or more constructors and lacks an `else`/`_` clause. Behaviour is
non-fatal — the match still runs, the warning goes to stderr.

- spec/evaluator.sx: helpers `match-clause-is-else?`, `match-clause-ctor-name`,
  `match-warn-non-exhaustive`, `match-check-exhaustiveness`. The latter
  reads the `*adt-registry*` (already populated by `define-type`),
  collects constructor patterns from clauses, and dedupes via an
  `*adt-warned*` env-bound dict so each (type, missing-set) warns once.
  Wired into `step-sf-match` via a `do` block before clause dispatch.

- hosts/javascript/platform.py: `host-warn` primitive (`console.warn`)
  + matching `hostWarn` js-id helper so the JS-transpiled spec code
  can call it directly. Spec code reaches JS via `sx_build target=js`.

- hosts/ocaml/lib/sx_runtime.ml + sx_primitives.ml: `host-warn` runtime
  helper (`prerr_endline`) and registered primitive.

- hosts/ocaml/lib/sx_ref.ml: HAND-PATCHED. `step_sf_match` now calls
  a hand-written `match_check_exhaustiveness` that handles both
  `AdtValue` and back-compat dict-shape ADT values. The OCaml side
  is *not* retranspiled because regenerating sx_ref.ml drops
  several preamble fixes (seq_to_list, string->symbol mangling,
  empty-dict literal bug). Future retranspile must reapply this patch.

- spec/tests/test-adt.sx: 5 new tests covering exhaustive,
  non-exhaustive (warning is non-fatal), `else` suppression,
  partial coverage with one missing constructor, and `_` wildcard
  suppression. Tests assert return values only — warnings go to
  stderr and are not captured.

Warning format: `[sx] match: non-exhaustive — TypeName: missing Ctor1, Ctor2`
Both hosts emit identical messages.

Tests: OCaml 4540 → 4545 (+5), JS 2586 → 2591 (+5). Zero regressions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_runtime.ml

@@ -411,6 +411,10 @@ let callcc_continuation_winders_len v = match v with
 let host_error msg =
   raise (Eval_error (value_to_str msg))
 
+let host_warn msg =
+  prerr_endline (value_to_str msg);
+  Nil
+
 let dynamic_wind_call before body after _env =
   ignore (sx_call before []);
   let result = sx_call body [] in